PAOs are typically produced by oligomerization of linear alpha olefins in the presence of boron trifluoride and a boron trifluoride-organic catalyst complex. The linear alpha olefins typically have carbon numbers ranging from about 6 to about 16. Typically, alpha-octene, alpha-decene, and alpha-dodecene are used either individually or as mixtures. The organic component of the boron trifluoride-organic catalyst complex comprise alcohols, acetates, acids or ethers having carbon numbers ranging from about 1 to about 10. These organic components are either used individually or as mixtures thereof. Typically, the organic component comprises ethanol, propanol, butanol, pentanol, the corresponding acetates and mixtures thereof. For example, boron trifluoride-butanol/butyl acetate is a catalyst complex.
Boron trifluoride constitutes a significant portion of the manufacturing raw material cost. As a result, it is desirable to minimize the overall usage of boron trifluoride by recycling the boron trifluoride-organic catalyst complex. Moreover, any boron trifluoride that is not recycled is eventually lost to the effluent system. It is desirable to reduce this form of boron and fluorine emissions.
Recycling the boron trifluoride-organic catalyst complex typically involves distilling the boron trifluoride from the crude PAO oligomer reaction mass under a vacuum. However, under the conditions of pressure and temperature in the distillation column, generally at least a portion the boron trifluoride-organic catalyst complex dissociates into boron trifluoride and the corresponding organic constituent(s). Under typical distillation conditions, due to poor contacting of boron trifluoride and the organic constituents, the resulting condensed boron trifluoride-organic catalyst complex is unsaturated. A boron trifluoride-organic catalyst complex is unsaturated when the molar ratio of the sum of the organic component, i.e., alcohols, acetates, acids and/or ethers, to boron trifluoride is greater than about 1:1. The complex is saturated when the molar ratio of alcohol to boron trifluoride is about 1:1. This unsaturation results in loss of free boron trifluoride.
Conventional approaches to improving boron trifluoride recovery and increase boron trifluoride-organic catalyst complex saturation by increasing the condenser residence time and also by using chilled water on the condenser are expensive and also not very efficient. There exists a need for an improved method of recovering boron trifluoride efficiently to reduce boron trifluoride usage and boron trifluoride emissions. There exists a need for an improved boron trifluoride recovery that reduces heat-induced degradation of the boron trifluoride-organic catalyst complex.